I think literally opening is meant (I could be wrong). Not just bacteria getting in is bad, even exposing product to oxygen (and light) is bad for it. Although I don't really believe if you open something just to smell it and don't use it otherwise, and it's stored away, it will go bad just because it was opened for a second. And then, there are some preservatives that have short shelf life, so whatever expiration date is indicated it could mean preservs expire by then...

I don't really pay attention to expiration dates, I use anything as long as there are no changes in consistency/scent/color. I only pay attention to how I store my products, I make sure they are away from light, heat and humidity, and everything is closed tightly. I never leave anything sitting there with a cap or lid off, and when I use something I close it immediately. When it's something in a jar I don't dip my fingers and use a spatula or spoon to get it out, always making sure it's dry; no water ever gets in into anything I use. I have many products that are a few years old, and expiration dates have long gone by, but because I stored them well they are still fine and I am using them with no problems. I have back ups of facial cleanser, cream, some oils and butters, and they are all in the freezer, everything that will have to wait a while until I get to it is there.

I just did a quick search, one of the first sites that came up was newsolio.com, and according to it, yeah, air is bad for products because germs and bacteria that are in the air get in them, and it also said to discard even unopened stuff after 3 years.